The present embodiments are directed to an imaging member used in electrostatography and a process for making and using the member. More particularly, the embodiments pertain to the preparation of an improved electrostatographic imaging member having low contact friction surfaces to ease sliding mechanical interaction and suppressing abrasion/wear failure. The improved imaging member has a slippery imaging layer, an additional low surface energy protective overcoat layer, and/or a reduced contact friction anti-curl back coating, each of which comprise one or two low surface energy polymeric materials that enhance the physical and mechanical functions of the imaging member to impart service life extension.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Electrostatographic imaging members are well known in the art. Typical electrostatographic imaging members include, for example: (1) electrophotographic imaging member (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring member which is used to remove the toner images from a photoreceptor surface and subsequently transfer these images onto a receiving paper.
Although the scope of the present disclosure covers the preparation of all types of electrostatographic imaging members in either a rigid drum design or a flexible belt configuration, for reasons of simplicity, the embodiments and discussion following hereinafter will be focused solely on and represented by electrophotographic imaging members in the flexible belt configuration. Electrophotographic flexible belt imaging members may include a photoconductive layer including a single layer or composite layers. The flexible belt electrophotographic imaging members may be seamless or seamed belts. The seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. Typical flexible electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anti-curl back coating coated onto the opposite side of the substrate layer. By comparison, a typical flexible electrographic imaging member belt has a more simple material structure, and it includes a dielectric imaging layer on one side of a flexible supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Since typical negatively-charged flexible electrophotographic imaging members exhibit undesirable upward imaging member curling after completion of coating the top outermost charge transport layer, an anti-curl back coating, applied to the backside, is required to balance the curl. Thus, the application of an anti-curl back coating is desirable to provide the appropriate imaging member with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a negatively-charged photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer of a positively-charged imaging member is sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer (CGL) is sandwiched between the outermost exposed charge transport layer (CTL) and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of Gen layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member web like photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
In either positively charged flexible imaging member belts or negatively charged flexible imaging member belts, an anti-curl back coating is usually used to counteract imaging member curling and maintain imaging member belt flatness.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of negatively charged multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a CGL, an outermost exposed CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anti-curl back coating (ACBC) on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, CTL and other layers. The CTL is usually the last layer to be coated to become the outermost exposed layer and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the CTL coating through drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL exhibits a larger dimensional shrinkage than that of the substrate support as the imaging member web stock (after through elevated temperature heating/drying process) as it cools down to ambient room temperature. The exhibition of upward imaging member curling after completion of CTL coating is due to the consequence of the heating/cooling processing, according to the mechanism: (1) as the web stock carrying the wet applied CTL is dried at elevated temperature, dimensional contraction does occur when the wet CTL coating is losing its solvent during 115° C. elevated temperature drying, because the CTL at 115° C. still remains as a viscous liquid after losing its solvent. Since its glass transition temperature (Tg) is about 85° C., the CTL will flow to automatically re-adjust itself to compensate the losing of solvent and maintain its dimension; (2) as the CTL in a viscous liquid state is cooling down further and reaching its Tg at 85° C., the CTL instantaneously solidifies and adheres to the CGL because it has transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) cooling down the solidified CTL of the imaging member web from 85° C. down to 25° C. room ambient will then cause the CTL to contract more than the substrate support since it has an approximately 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This dimensional contraction mis-match between these two coating layers results in tension strain built-up in the CTL, at this instant, is pulling the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling effect, an ACBC is applied to the backside of the flexible substrate support, opposite to the side carrying the photo electrically active CTL/CGL, and render the imaging member web stock with desired flatness.
Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An ACBC, having a counter curling effect to balance the applied photo electrically active layers, is applied to the opposite or back side of the support substrate to maintain the overall photoreceptor flatness by offsetting the curl effect which is arisen from the mismatch of the thermal contraction coefficient between the substrate and the CTL, resulting in greater CTL dimensional shrinkage than that of the substrate. However, common ACBC formulations do not always provide satisfying dynamic photoreceptor belt performance result under a normal machine functioning condition. For example, exhibition of ACBC wear and its propensity to cause tribo-electrical charging up are frequently seen problems that prematurely cut short the service life of a belt and requires frequent costly replacement in the field. ACBC wear reduces the thickness and thereby diminishes its anti-curling capacity. Moreover, ACBC tribo-electrical charge up against belt support module rollers and backer bars is very problematic since it increases the torque for effective belt drive to the point (in some occasions) causing total belt stalling under the dynamic belt cycling machine operation condition.
Other layers of the imaging member, for example the top outermost exposed CTL in a negatively charge imaging member, also suffer from the machine operational conditions, such as exposure to high surface friction and extensive cycling. Such harsh conditions lead to abrasion, wearing away, and susceptibility of surface scratching of the CTL which otherwise adversely affect machine performance. Another imaging member functional problem associated with the CTL is its propensity to give rise to early development of surface filming due its high surface energy. CTL surface filming is undesirable because it pre-maturely causes degradation of copy printout quality. Moreover, the outermost exposed CTL has also been found to exhibit early onset of surface cracking, as consequence of repetition of bending stress belt cyclic fatiguing, airborne chemical species exposure, and direct solvent contact, under a normal machine belt functioning condition. CTL cracking is a serious mechanical failure since the cracks manifest themselves as defects in print-out copies. All these imaging member layers failures remain to be resolved.
In U.S. Pat. No. 5,069,993, which is hereby incorporated by reference in its entirety, an exposed layer in an electrophotographic imaging member is provided with increase resistance to stress cracking and reduced coefficient of surface friction, without adverse effects on optical clarity and electrical performance. The layer contains a polymethylsiloxane copolymer and an inactive film forming resin binder. Various specific film forming resins for the anti-curl layer and adhesion promoters are disclosed.
U.S. Pat. No. 5,021,309, which is hereby incorporated by reference in its entirety, shows an electrophotographic imaging device, with material for an exposed anti-curl layer has organic fillers dispersed therein. The fillers provide coefficient of surface contact friction reduction, increased wear resistance, and improved adhesion of the anti-curl layer, without adversely affecting the optical and mechanical properties of the imaging member.
U.S. Pat. No. 5,919,590, which is hereby incorporated by reference in its entirety, shows an electrostatographic imaging member comprising a supporting substrate having an electrically conductive layer, at least one imaging layer, an anti-curl layer, an optional ground strip layer and an optional overcoat layer, the anti-curl layer including a film forming polycarbonate binder, an optional adhesion promoter, and optional dispersed particles selected from the group consisting of inorganic particles, organic particles, and mixtures thereof.
In U.S. Pat. No. 4,654,284, which is hereby incorporated by reference in its entirety, an electrophotographic imaging member is disclosed comprising a flexible support substrate layer having an anti-curl layer, the anti-curl layer comprising a film forming binder, crystalline particles dispersed in the film forming binder and a reaction product of a bifunctional chemical coupling agent with both the binder and the crystalline particles. The use of VITEL PE 100 in the anti-curl layer is described.
In U.S. Pat. No. 6,528,226, which is hereby incorporated by reference in its entirety, a process for preparing an imaging member is disclosed that includes applying an organic layer to an imaging member substrate, treating the organic layer and/or a backside of the substrate with a corona discharge effluent, and applying an overcoat layer to the organic layer and/or an anti-curl back coating to the backside of the substrate.
The above disclosures show that, while attempts to resolve charge transport layer and anti-curl back coating problems have been made, those solutions do not address all the additional problems that arise. Therefore, there is a need to provide improved imaging members that have mechanically robust outer layers to effect service life extension but without causing the introduction of other undesirable problems.
To resolve these physical/mechanical associated problems and effect the imaging member service life extension, the present embodiments provide: (1) slippery CTL formulation, (2) addition of an low surface energy overcoating layer and (3) a low friction ACBC design. The improved imaging member of this disclosure, as described and detailed in the embodiments presented hereinafter, addresses the shortcomings of traditional imaging layers discussed above and specific improvements to provide physical/mechanical robust functions to the imaging member.